Switched mode power supplies are a common form of voltage regulator that is widely used because of its higher efficiency than linear regulators. This is especially true when there is a wide disparity between input and output voltages. For example, in an automotive environment, the input voltage can go as high as 40 volts whereas the output voltage may be only 1.8 volts to power an integrated circuit, for example. The utilization of a switch mode power supply for this purpose not only reduces the energy consumption from the regulator, but also produces a concurrent reduction in the amount of heat that needs to be dissipated.
FIG. 1 illustrates a known type of switched mode power supply suitable for this purpose, for example, generally shown as 100. This regulator has a switching transistor 102 coupled between a source of power and an input to an inductor 104. A capacitor 108 is coupled between the output of the inductor and ground and a free wheeling diode 106 is coupled between the input to the inductor and ground, as is well known in the art. The switching transistor 102 utilized in the regulator 100 in the shown embodiment is a NMOS transistor. Accordingly, a level shift in circuit 116 is utilized to produce a higher voltage than the input voltage in order to produce the proper drive for an NMOS transistor. In this particular case, the output of a voltage regulator 112 is utilized by a bootstrap circuit 114 and a capacitor 110 coupled between the bootstrap circuit and the input to the inductor 104, in an known matter. The output of the bootstrap circuit 114 is utilized to produce the higher voltage in the level shifting circuit 116 to generate the proper drive for the NMOS transistor 102.
The switching transistor 102 is driven in accordance with signals generated by the logic core circuit 118 which responds to the output of an error comparator 132 comparing a ramp signal 136 generated by ramp generator 138 with the output of error amplifier 122. The ramp signal 136 is generated by charging a capacitor utilizing the reference voltage VBG, which may be a bandgap voltage, for example. The capacitor is then discharged in accordance with the clock signal applied to the input of the ramp generator 138, as is well known in the art. The ramp signal is compared by 132 with the output of error amplifier 122. Error amplifier 122 compares a portion of the output voltage VOUT via a tap 126 on resistor divider 124 with a reference voltage VREF to produce error voltage on line 130 coupled to the inverting input of comparator 132. The times when the ramp voltage exceeds the error voltage determines when the switching transistor 102 will be turned on. The error amplifier 132 is compensated by compensation circuit 120 which comprises three capacitors and three resistors, two of which are the resistor chain in the resistor divider 124, to assure stability of. the amplifier 122. In many designs, the compensation network and the resistor divider are outside of the integrated circuit and are therefore comprise discrete components. There is a desire to integrate these components within the integrated circuit in order to reduce the size and cost of the circuit. As is well known to those skilled in the art, integrated capacitors take up a substantial portion of the real estate of an integrated circuit chip. Therefore, there is a desire to make these capacitors as small as possible. This means, in order to maintain the time constants, the resistors must be increased in value. Higher value resistors take up much less room on the integrated circuit than higher value capacitors, so that this is a good trade off with respect to saving the real estate. on the integrated circuit chip. However, the disadvantage of this approach is that the input node VFB becomes a relatively high impedance node and, during transient events, does not closely follow the output node. This results in poor regulation during of these events.
A known solution to this problem is shown in FIG. 2 generally as 200. In FIG. 2, elements shown in FIG. 1 have similar reference numerals to the reference numerals in FIG. 1. In FIG. 2, a hysteretic comparator 240 is added to the circuit shown in FIG. 1. The inverting input of this comparator is coupled to the output voltage VOUT and the noninverting input is coupled to a second reference VREF2. The output of the hysteretic comparator 240 on line 244 is coupled to a separate control circuit 246 within the logic core 218 that controls the switching of transistor 202. When the output voltage falls below a preset voltage (under voltage) the high side switching transistor 202 is held on ignoring the output of the error amplifier 222. When the output voltage reaches a second predetermined voltage, higher than the first predetermined voltage hysteretic signal to, the high side switching transistor turns off so that the main control loop including ramp generator 238, error amplifier 222 and comparator 232 can take over. This solution provides a faster response to a transient then the solution shown in FIG. 1. However, the official loop is added in parallel to the main loop and this may make it difficult to compensate the main loop and in addition the hysteretic comparator might not let go of it's control after the transient event has terminated. In addition, because of external inductor variations, it is difficult to identify at the precise moment that the hysteretic comparator should be turned on or off, which may cause the output voltage to undershoot when the comparator turns the transistor on slightly slower. If the switching transistor 202 is not turned on in the right moment, the output voltage may overshoot it's intended value and that timing is dependent upon the current through the external inductor. Accordingly, more complex methods including forcing the on and off times of the transistor 202 may have to be added to the circuit which not only slow down the transient response of the regulator but greatly increased it's complexity.
The operation of the hysteretic control loop is shown in FIG. 3 generally as 300. As shown by the waveform 302, when the output voltage drops below a predetermined voltage VREF3 the hysteretic control will turn the switching transistor 202 on as shown at 304. However, when the output voltage exceeds the voltage reference VREF3, the transistor stays on until the voltage increases to a second predetermined voltage VREF2 and shown as 306.
Accordingly, there is a need for a switched mode power supply which can use integrated components for both the compensation and the voltage divider and still maintain fast transient response.